Transistor line switch



March 14, 1961 P|NET 2,975,305

TRANSISTOR LINE SWITCH Filed Aug. 12, 1959 ave/v41. w s/a/vAL CIRCUIT 33 8 32 2 29 CIRCUIT INVENTOR; Andre Eugene Pine! All TRANSISTOR LINE SWITCH Andre Eugene Pinet, St. 'Maur-des-Fosses (Seine), France,

assignor to Automatic Electric Laboratories, Inc., a corporation of Delaware Filed Aug. 12, 1959, Ser. No. 833,126

Claims priority, application France Sept. 26, 1958 8 Claims. (Cl. 30788.5)

The present invention relates to electronic switches capable of establishing and cutting an electrical trunk between two networks such, for example, as automatic telephone networks. These switches have two positions, an open position and a closed position.

Switches of this type are known comprising a modulator placed under the control of a trigger circuit in such a way that the modulator is blocked in one state and unblocked in the other state of the trigger circuit. The opening or the closing of the switch depends therefore on the state of the trigger circuit, which may be controlled by the transmission of a pulse signal of appropriate polarity to an appropriate terminal of the trigger circuit.

Bistable trigger circuits are known having a right side and a left side. For each of the stable states, one of the sides, such as the right for example, is blocked and the other side, such as the left for example, is passing. In these trigger circuits, one of the sides is conducting regardless of the state of the trigger circuit; the power feed source for the trigger circuit therefore feeds current con tinuously.

An object of the present invention is the provision of an electronic switch of this type having a simpler arrangement than that of the switches of the prior art.

Another object of the invention is a switch of this type in which the trigger circuit, in one of its states, blocks the power source which feeds it in such a way that the latter does not feed current while the trigger circuit is in the said state. In this manner, if the trigger circuit is normally in one state more than the other, for example in the state corresponding to the open position of the switch, a considerable economy results if this state corresponds to that which blocks the power feed source. 7

' Another object of the invention is a bistable switch in which the passage from one state to another is at the same time under the control of two triggering signals, and does not change state, for example, unless both signals are simultaneously present.

According to a feature of the invention, one of the sides of the trigger circuit itself serves as a modulator for the switch. When this side is blocked the switch is open; when this side is conducting, the switch is closed.

According to another feature of the invention, the modulator-controlling trigger circuit comprises two transistors of opposite type, interconnected according to the known arrangement for transistors of the same type, but powered with an appropriate inversion of polarity between one side and the other side. Under these conditions, in one of the stable states of the trigger circuit, the two transistors are blocked; in the other stable state, the two transistors are conducting. When the trigger circuit is in the first stable state, the power feed sources for this trigger circuit feed practically no current.

The invention will be better understood upon reading the detailed description which is to follow, and upon examining the single annexed figure which represents the "ice tive polarization with respect to ground. The terminal 4 of the collector of this transistor 1 is connected by way of the resistance 13 to a source of direct current 12 providing a positive polarization with respect to ground. By

way of resistance 23, this same terminal 4 is connected to the terminal 16 of the base of transistor 14, which is of the type p-n-p.

The terminal 15 is of the emitter and the terminal 16 of the base of transistor 14 are connected by way of the resistances 19 and 20 respectively to a source of direct current 18 providing a positive polarization with respect to ground. The terminal 17 of the collector of this same transistor 14 is connected by way of the resistance 22 to a source of direct current 21 providing a negative polarization with respect to ground. By way of the resistance 24, this same terminal 17 is connected to the terminal 3 of the base of transistor 1.

The source 12 gives a positive potential superior to that of the source 18, and the source 21 gives a negative potential superior to that of the source 9.

The resistances 23 and 24 couple the transistors 1 and 14 to constitute an electrical device having the two desired stable states of equilibrium. For the first state of equilibrium the two transistors are simultaneously blocked. For the second state of equilibrium the two transistors are simultaneously conductive.

The switching circuit of the invention is in an unstable state when on of the transistors of the device is blocked and the other transistor unblocked. The said unstable state can exist only under the influence of external signals, or during the period of transition from one stable state to the other.

If we consider the state in which the two transistors 1 and 14 are both blocked, the potential of the base of transistor 1 has a value which is intermediate between that of the emitter connected to the source 9, and that of the source 21, by the interplay of the resistances 11, 24 and 22. It follows that the potential of the base is negative with respect to the potential of the emitter, since the source 21 has a negative voltage greater than that of the source 9. The transistor 1 being of the type n-p-n, the polarization applied to its base has therefore the efiect of keeping it blocked.

In like manner, for the p-n-p type transistor 14, its base is maintained, by the interplay of the resistances 20, 23 and 13 at a positive potential of a higher value than that of the emitter, since the source 12 has a positive voltage greater than that of the source 18. The polarization applied to the base has therefore the elfect of keeping the said transistor blocked.

Considering now the case of the second stable state in which the two transistors 1 and 14 are conducting, let it be assumed temporarily that the emitter resistances 10 and 19 are not present. When the transistor 1 is conductive, the potential of its collector is substantially equal to that of the source of negative polarization.9 by virtue of the voltage drop caused by the collector current in the resistance 13. This negative potential is transmitted to the base of the transistor 14 through the resistance 23. The base is thus driven to a negative value with respect to the source 18, and in consequence, with respect to the emitter. The result of this is that the transistor 14 is unblocked and brought into 'its saturation zone. The

- potential of the collector therefore attains a positive value approaching that of the source 18. The stated positive potential, transmitted in its turn to the base of the transistor 1 through the resistance 24, has the elfect of polarizing this base positively with respect to the emitter. The transistor 1 is accordingly in this manner maintained in the zone of saturation.

Practically, to maintain the transistors 1 and 14 in the region of saturation, it is not necessary to apply polarizations of such high values between their emitters and their bases. The insertion of the resistances l and 19 in the emitter circuits, while reducing the polarization voltage, does not change the foregoing operation. Nevertheless, it is necessary that the resistances and 19 have low values with respect to those of the collector resistances 13 and 22.

The passage from one stable state to the other stable state may be efiected by applying a control signal of appropriate polarity to the terminal 25 connected to the terminal 3 through the condenser 26.

If for example, the two transistors 1 and 14 are initially in the blocked state, it is necessary to apply a positive impulse to the terminal 25 to make the said transistors conductive. This positive impulse, if it is of sufficient amplitude, has for efiect, since it is transmitted to the base of transistor 1, to carry the said base to a positive potential with respect to the emitter. Transistor 1 then becomes conductive and the potential of its collector takes a value approximating that of the source 9. It has already been shown that under these conditions, by the action of the resistances 23 and 20, the transistor 14 is also brought into the saturation zone. It has likewise been shown that the resistance 24 then swings the terminal 3 to a positive potential with respect to the emitter of the transistor 1, so that even if the signal applied to the terminal 25 disappears the transistors will remain in the conductive state.

In order for the transistors 1 and 14 to return to the blocked state, we may for example, apply a negative impulse to the terminal 25, which impulse, transmitted to the terminal 3, has the effect of blocking transistor 1. The triggering operation opposite to that which has just been described then takes place, and at the end of a very short time, the two transistors of the switching circuit are blocked. This blocking of course, is maintained after the removal of the negative impulse applied to the terminal 25.

The terminal is connected by way of the condenser 27 to one of the input terminals 28 of the electrical circuit 29 (a telephone circuit for example) which is to be connected by way of the switching circuit to another electrical circuit 33, one of whose input terminals 32 is connected to the terminal 17 by way of a diode 31. The two other input terminals 30 and 34 belonging respectively to the two circuits 29 and 33 to be interconnected are connected directly to each other, and are preferably grounded. The diode 31 is connected in such a way that its passing orientation is from the terminal 17 to the input terminal 32 of the electrical circuit 33.

It will now be shown how the joining or separation of the two electrical circuits 29 and 33 is effected by means of the switching circuit of the invention.

If it is assumed that the transistors 1 and 14 are blocked, the terminal 28 of the circuit 29 is practically insulated electrically from the terminal 32 of the circuit 33. In effect, when the two transistors 1 and 14 are blocked, the impedance of the switching circuit between the terminal 15 and the terminal 17, through the transistor 14, has a very high value. Furthermore, the resistance of the diode 31 is likewise very high, since this diode is polarized in a direction to block it, because of the fact that on the one hand, the anode of this diode is connected to the terminal 17 which is practically at the negative potential of the source 21, while on the other hand, the cathode of the diode is at ground potential by way of the circuit 33.

The result of the foregoing considerations is that a telephone signal for example which may eventually be present in the circuit 29 cannot be transmitted to the circuit 33 when the two transistors 1 and 14 are blocked. Similarly, a telephone signal eventually present in the circuit 33 cannot be transmitted to the circuit 29 because of the high attenuation factor of the T section whose two horizontal arms are composed respectively of the inverse resistance of the diode 31 and the resistance of the transistor 14, and whose vertical arm is composed of the resistance 22.

It should be noted that the presence of a telephone signal, either in the circuit 29 or in the circuit 33, has no eiiiect on the state of the transistors in the switching circuit of the invention, provided that the amplitudes of the crests of the telephone signal do not reach maximum values completely nullifying the efiects of the inverse polarizations applied between the bases and the emitters of the transistors of the switching circuit.

When a positive impulse is applied to the terminal 25 of the switching circuit of the invention, the two transistors are brought to their saturation zones. The result of this is that the impedance presented by the transistor 14 between its emitter and its collector is very low; in addition, the resistance presented by the diode 31 is likewise low, since this diode is in this case polarized in its passing direction. In fact, its cathode is always connected to ground potential through the circuit 33, but its anode is carried to a potential approximating that of the source of positive polarization 18. The terminal 28 of the circuit 29 is therefore connected, through a very low resistance, to the terminal 32 of the circuit 33. An electrical continuity is thus assured, by means of the switching circuit of the invention, and the telephonic signals are transmitted from one circuit to the other with a negligible attenuation.

It will be very evident that this state of electrical continuity is assured only if the crests of the telephone signals do not nullify the eifects of the unblocking polarizations applied to the transistors of the switching circuit.

The table following indicates, by way of example, particular values for the potentials of the different sources of direct current, as well as for the various resistors and condensers of a switching circuit according to the invention, which have given satisfactory operating results. These values are given only by way of example, and the invention is not limited to the said values, nor to certain of them.

Resistance 10 0hms Resistance 11 do 27,000 Resistance 13 do 3,900 Resistance 23 do 5,600 Resistance 19 do 600 Resistance 20 do 27,000 Resistance 22 do 5,600 Resistance 24 do 15,000 Condenser 26 microfarads 0.1 Condenser 27 do 1.0 Battery 9 volts 6 Battery 12 do 12 Battery 18 do 6 Battery 21 do 12 Transistor 1 n-p-n Type 2N94 Transistor 14 p-n-p Type 2N43 Diode 31 Germanium, normal type The performances obtained with a switching circuit of the invention constructed according to the foregoing specifications, are as follows:

(a) Attenuation between the terminals 28 and 32 when the transistors 1 and 14 are conducting: 0.08 neper.

(b) Attenuation between the terminals 28 and 32 when the transistors 1 and 14 are blocked: 8 nepers.

(c) Amplitude of the maximum permissible crest for the telephone signals: 3 volts.

(d) Amplitude of the control signals: 12 volts.

(e) Minimum duration of the application of the control signals: microseconds.

Although there has been shown and described a particular mode of construction specific to the present invention, a person skilled in the art will readily understand that certain variants may be employed.

One of these variants consists in adding to the circuit, as shown by the single figure, a fourth terminal 35 connected to the emitter of the transistor 1. This terminal 35 may receive control signals. The triggering signals applied to the switching circuit of the invention may in this manner be divided in two in such a way for example, that in order to make the transistors 1 and 14 conductive, it will be necessary to apply a positive impulse to the terminal and a negative impulse to the terminal simultaneously. These two impulses, whose efiects are cumulative, may have appropriate values so that the presence of only one of them will be insufiicient to unblock transistor 1. This latter four-terminal arrangement is particularly interesting in the case where matrix type connection networks are employed.

What is claimed is:

l. A switching circuit for opening and closing an alternating current line extending between two signal circuits, a common reference point connected to both of said signal circuits to form one conductor of said line, said switching circuit comprising a transistor having an emitter electrode, a collector electrode and a base electrode, a diode connected between said collector electrode and one of said signal circuits, and a capacitor connected between said emitter electrode and the other of said signal circuits, another conductor of said line extending between said signal circuits through the diode, the transistor, and the capacitor; a bistable trigger circuit including said transistor and direct-current bias means coupling each of said electrodes to the reference point, said trigger circuit having one stable state in which said transistor is conducting and another stable state in which said transistor is non-conducting; a direct-current path extending from the collector electrode through the diode and one of the signal circuits to the reference point, the diode being so poled that with the trigger circuit in the state having the transistor conducting the diode is forward biased, and with the trigger circuit in its state having the transistor nonconducting the diode is reverse biased, and means independent of any potential on said line for applying control signals to said trigger circuit for alternately triggering the trigger circuit to its said two stable states; whereby with the trigger circuit in the state having the transistor conducting a low impedance path exists for alternating current signals on said line, and with the trigger circuit in the state having the transistor non-conducting a high impedance circuit effectively blocks alternating current signal flow over said line.

2. A switching circuit for opening and closing an alternating current line extending between two signal circuits, a common reference point connected to both of said signal circuits to form one conductor of said line, said switching circuit comprising a first transistor of one conductivity type and a second transistor of opposite conductivity type, each of said transistors having an emitter electrode, a collector electrode and a base electrode; a diode connected between the collector electrode of the first transistor and one of said signal circuits, and a capacitor connected between the emitter electrode of the first transistor and the other of said signal circuits, another conductor of said line extending between said signal circuits through the diode, the transistor, and the capacitor; a bistable trigger circuit including both said transistors, direct-current bias means which includes connections to all said electrodes of both said transistors,

and a cross-coupling path interconnecting the base electrode of each transistor with the collector electrode of the other transistor; said trigger circuit having one stable state in which both transistors are conducting and another state in which both transistors are non-conducting; a direct-current path extending from the collector electrode through the diode and one of the signal circuits to the reference point, the diode being so poled that with the trigger circuit in its conducting state the diode is forward biased, and with the trigger circuit in its non-conducting state the diode is reverse biased, and means independent of any potential on said line for applying control signals to said second transistor for alternately triggering said trigger circuit to the conducting stable state and the nonconducting stable state.

3. A switching circuit according to claim 2, wherein said cross-coupling paths comprise a resistor connecting the base electrode of each transistor directly to the col- 'lector electrode of the other transistor, one of said tram sistors' is p-n-p type and the other is n-p-n type, said bias means includes a common reference point, a first directcurrent source of negative polarity connected between the reference point and the emitter electrode of the n-p-n transistor, a second direct-current source of positive polarity and a resistor connected in series between the reforcnce point and the collector electrode of the n-p-n transistor, a third direct-current source of positive polarity connected between the reference point and the emitter electrode of the p-n-p transistor, a fourth direct-current source of negative polarity connected in series with a resistor between the reference point and the collector electrode of the p-n-p transistor, a resistor connected between the base electrode of the n-p-n transistor and the first source, and a resistor connected between the base eleoof the p-n-p transistor and the third source, the fourth source being of greater potential than the first source and the second source being of greater potential than the third source, whereby when the transistors are non-conducting the base electrode of the n-p-n transistor is negative and the base electrode of the p-n-p transistor is positive with respect to their respective emitter electrodes, and whereby when the transistors are non-conducting the current drain from said sources is negligible.

4. A switching circuit according to claim 3, wherein a resistor is connected between the emitter electrode of each transistor and its emitter bias source.

5. A switching circuit according to claim 3, wherein the said first transistor is the p-n-p type, and the said second transistor is the n-p-n type.

6. A switching circuit according to claim 2, wherein said means for applying control signals comprises two separate input terminals coupled to said second transistor for triggering said trigger circuit responsive to two coincident signals applied respectively to said input terminals.

7. A switching circuit according to claim 6, wherein said two input terminals are coupled to the base electrode and the emitter electrode respectively of said second transistor.

8. A switching circuit according to claim 2, wherein said first transistor is of p-n-p type, and said diode has its anode connected to the collector electrode of the first transistor.

Referenem Cited in the file of this patent UNITED STATES PATENTS 2,605,306 Eberhard July 29, 1952 2,644,896 1.0 duly 7, 1953 2,823,322 Trousdale Feb. 11, 1958 2,853,631 Wallace Sept. 23, 1958 2,907,895 Over-beck Oct. 6, 1959 

